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Tube Compressor issues

JCLAY85

JCLAY85

New Member
Hi All,
I've been working hard to try and get my Drip Opto 7 (stereo LA2A clone) up and running. I posted yesterday about the power issue (which I got resolved). Its working for the most part, but I have 4 issues that I would surely appreciate any help with and thought I'd see if anyone could point me in the right direction!

1. Hum
Sounds maybe like ground loop hum? Its lower in pitch, like if you grab a guitar cable that's coming from an amp. I hear it even when the gain is all the way down. The volume of the hum doesn't change. Powering the unit off makes the noise disappear. Its not super loud, but definitely noticeable. HAVE TRIED: disconnecting pin 1 of both the XLR outputs. Am I missing something here? I tried to keep wiring as neat as possible, twisting power cable and running them to the outside corner. See photo for my wiring.

2. Bypass switch not working
I may not have this hooked up right - there isn't much documentation from drip regarding hooking up the bypass. Other than directing that at least one of the GND pads needs to be grounded. I grounded 2.

3. LED lamps
My LED's in my VU meters appear to have burned out. Inside the VU meter there are two LED wired in series. Im getting 6.3 volts across the LED terminals. If I stick an led across the terminals, it lights up, but quickly gets very hot. The LED's were working, but are no longer lighting up. Any Idea what would cause this?

4. Incorrect voltage at R13 resistor.
On both channels, I'm getting around 180v instead of the prescribed 120 at the R13 resistor. Any ideas here? I'll attach a schematic so you can see which one it is.

Thank you in advance for any guidance here! It is GREATLY appreciated.

Photos/diagram/schematic of my unit uploaded here:
Dropbox - GroupDiy LA2A Drip opto 7 photos
 
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1) Check power supply and capacitors. Usually hum independent of the volume control is power supply related. If it changed with the volume control then it wood most likely be a ground loop issue.
2) I hate those things.:D I get headaches looking at those hookup diagrams.
3) Maybe the LED's were lower voltage.
4) Haven't looked at the schematic. Honestly, I just didn't want to get too involved. Doing 10 different things at the moment.
 
About 4. What is the cathode voltage at R14? Schem says 1.1 vdc and if you are close to that I wouldn’t worry.

You can also confirm the plate load resistor is indeed 220K and that the Rk is 2.7K.

The power supply voltage might be worth measuring too. Ahead of the plate loads you have a dropping resistor 68K and an expected voltage of 275.

The AC input on that schem is 117V so if you have one, dial your variac back to 117vac and then measure.

3. LEDs need a current limiting resistor. See this calculator: http://led.linear1.org/1led.wiz
 
RWood said:
About 4. What is the cathode voltage at R14? Schem says 1.1 vdc and if you are close to that I wouldn’t worry.

You can also confirm the plate load resistor is indeed 220K and that the Rk is 2.7K.

The power supply voltage might be worth measuring too. Ahead of the plate loads you have a dropping resistor 68K and an expected voltage of 275.

The AC input on that schem is 117V so if you have one, dial your variac back to 117vac and then measure.

3. LEDs need a current limiting resistor. See this calculator: http://led.linear1.org/1led.wiz
Click to expand...


R14 is. measuring 1.1mV on one side and 1.3 on the other. Could that be a typo (v=mv?) in the schem? Seems very coincidental.

Out of the power supply I'm getting 271v. Im just measuring the voltage outputs right where they come from the power supply. Is that the right place?
 
century tek said:
1) Check power supply and capacitors. Usually hum independent of the volume control is power supply related. If it changed with the volume control then it wood most likely be a ground loop issue.
2) I hate those things.:D I get headaches looking at those hookup diagrams.
3) Maybe the LED's were lower voltage.
4) Haven't looked at the schematic. Honestly, I just didn't want to get too involved. Doing 10 different things at the moment.
Click to expand...

From what I can tell, the power supply seems to be functioning as per normal. It seems to be outputting the proper voltage. What should I be checking exactly?
 
The voltage from the cathode pin 8 to ground should be 1.1 volts, not millivolts.

With the unit turned off and caps drained measure resistance from pin 3 to ground and see how close to 2700 ohms it is.

I am wondering if there is a wrong value resistor in there like 270K.
 
From the schematic you posted in the Dropbox,
R14 is a cathode resistor 2.7K. It connects pin 8 (not pin 3 as i said before) to ground.

R12 is grid resistor 470K to ground.

Regardless of layout or schematic, look for empirical evidence by measuring the resistance from pin 8 to ground.

If you mistakenly put a 470K in there instead of 2.7K you’d get the results you describe - a triode that is barely conducting and because of that, a plate voltage that is way too high. It is easy to measure, and also easy to fix.
 
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